Phosphatases are a group of enzymes capable of dephosphorylating or phosphorylate a substrate, i.e., the enzyme hydrolyzes phosphoric acid monoesters into a phosphate ion and a molecule with a free hydroxyl group or vice versa.
Ectophosphatases are a subclass of phosphatases which function extracelluarly, i.e., are capable of dephosphorylating an extra-cellular substrate in the extra-cellular space. This in contrast with intracellular phosphatases (also designated as kinases) (de)phosphorylating an intra-cellular substrate inside the cell, i.e., the intracellular space. The intra-cellular phosphatases are often involved in signal transduction.
Ectophosphatases can be in the form of integral membrane or GPI-anchored proteins displaying their catalytic domain, i.e., the domain involved in the actual dephosphorylating of a substrate, to the extra-cellular space. As an alternative, ectophosphatases can be present in the extra-cellular space as secreted or soluble proteins.
Ectophosphatases, and especially alkaline phosphatases (also designated in the art as AP, ALP or APhos), have been reported to be implicated in attenuation of inflammatory insults through their phosphatase activity on substrates such as, amongst others, endotoxins and nucleotides. Other ectophosphatases, like CD39 and CD73 (nucleotidases, apyrases), have been implicated in prevention of thrombolysis.
Alkaline phosphatase (ALP) (EC 3.1.3.1) is a hydrolase enzyme responsible for removing phosphate groups from many types of molecules, including nucleotides, proteins, and alkaloids. As is indicated by the name, alkaline phosphatases are most effective in an alkaline environment.
In humans, ectophosphatases, like alkaline phosphatase, are present in all tissues throughout the entire body, but are particularly concentrated in liver, bile duct, kidney, bone, and placenta.
Known species of alkaline phosphates are, for example, Bacterial alkaline phosphatase (BAP), Shrimp alkaline phosphatase (SAP), Calf intestine alkaline phosphatase (CIAP), Bovine intestinal alkaline phosphates (bIAP), and Placental alkaline phosphatase (PLAP) and its C terminally truncated version that lacks the last 24 amino acids (constituting the transmembrane domain)—the secreted alkaline phosphatase (SEAP).
Human alkaline phosphatises are catagorised as tissue nonspecific alkaline phosphatises (also referred as TNSAP or bone/liver/kidney type) and tissue specific alkaline phosphatises (placental/intestinal and germ cell type). To the TNSAP's also belongs, for example, alkaline phosphate present in milk and expressed by white blood cells.
Ecto-nucleoside Triphosphate Diphosphohydrolase 1, also designated in the art as CD39 or apyrase, is a nucleotide metabolizing enzyme belonging to a family of acid anhydride hydrolases. Examples of other enzymes belonging to this family are GTP phosphohydrolase, pyrophosphatase and thiamin-triphosphatase.
The ectophosphatase was first identified in 1949 and in 1963 partially purified from potato. The enzyme is also known under its registry number EC 3.6.1.5.
Apyrases are naturally occurring transmembrane glycoproteins that can activate intracellular pathways upon activation. Apyrases are found in a large number of microbial species such as E. coli, Aspergillus fumigatus and Kluyveromyces lactis, plants such as Arabidopsis thaliana, Glycine max and Oryza sativa, insects such as Drosophila melanogaster and mammals like Rattus norvegicus, Mus musculus and Homo sapiens. 
An apyrase enzyme comprises three domains, an extracelluar, a transmembrane and an intracellular domain. The extracellular domain comprises a conserved catalytic region responsible for the catalytic activity of the extracellular enzyme.
The catalytical domain catalyzes the hydrolysis of ATP to yield AMP and orthophosphate. Such activity can thus be characterized as an ATP-diphosphatase or ATP diphosphohydrolase. It can also act on ADP, again yielding AMP and orthophosphate. This activity can be characterized as an ADPase or ADP phosphohydrolase. Based on the combined enzymatic activities of the catalytic domain, the enzyme can also be regarded as an ATP-ADPase.
Reported physiological functions of apyrases address their possible involvement in maintenance of haemostasis and inhibition of platelet aggregation through hydrolysis of extracellular ADP, which is released from activated thrombocytes upon vascular injury.
CD73, also an ectophosphatase, converts monophosphate nucleotides like AMP to nucleosine+phosphate. (Nucleosine is non-inflammatory, whereas ATP and ADP and to lesser degree AMP are pro-inflammatory moieties, once presented extracellularly.